1. Field of the Invention
The present invention relates to a bidirectional transmission system with identical laser components.
The preferred field of application of the invention is in wave-division multiplexing (WDM). This procedure uses narrow optical filters as demultiplexers.
2. Discussion of the Background
There have been remarkable advances in optical communications due to the arrival of semiconductor lasers, which are easily amplitude or frequency-modulatable light sources.
Thus, conventionally an optical transmission system comprises a modulated semiconductor laser, a transmission line (such as an optical fiber) and a photoreceiver able to demodulate the optical signal transmitted by the line.
Recently, photoreceivers have been developed, which are particularly suitable for such links. They are laser structures identical to those used for forming emitter lasers, but which operate under special conditions and in particular below the laser effect threshold.
A description of such photoreceivers is provided in:
The article by S. Kobayashi et al entitled "Automatic Frequency Control in a Semiconductor Laser and an Optical Amplifier" published in "Journal of Lightwave Technology", Vol. LT-1, No. 2, June 1983, pp. 394-401.
French patent application FR-A-2 635 423 entitled "Process and apparatus for the optical filtering and photodetection of intensity-modulated optical signals".
The article by K. Magari et al entitled "Optical Signal Selection with a Constant Gain and a Gain Bandwidth by a Multielectrode DFB Laser Amplifier", published in "Applied Physics Letters", 1987, 51, pp. 1974-1976.
The article by T. L. Koch et al entitled "Tunable Multiple-Quantum-Well Distributed-Bragg Reflector Lasers as Tunable Narrowband Receivers", published in "Electronics Letters", Jul. 6, 1989, Vol. 25, No. 14, pp. 890-892.
In the penultimate reference, a description is given of an optical filter tunable to 20.6 GHz and with a bandwidth of 2.9 GHz and formed by a distributed feedback semiconductor structure polarized just below the threshold.
In the final reference a description is given of a tunable resonant amplifying photodetector (TRAP) constituted by a laser structure of the distributed Bragg reflector type and having multiple quantum wells (MQW). When this structure operates at above the threshold, it behaves like a tunable emitter and when it is polarized below the threshold, it is able to receive and demodulate an optical signal. Thus, it was possible to form a 250 mbit/s link with two identical components, the upstream component operating as an emitter and the downstream component as a receiver.
Although the optical links obtained with such components are advantageous in that they only require a single semiconductor structure usable both in emission and reception, they suffer from the disadvantage of their unidirectionality (i.e. they only operate in one direction).